An automatic transfer switch (ATS) usually switches power to an output from a first power source to a second power source, through turn off a plurality of relays which are coupled to the first power source, and turn on a plurality of relays coupled to the second power source. However, when the plurality of relays which are coupled to the first power source are turned off instantly, arc discharge will be occurred, which will not only lead to the first and second power source short circuited, but also lead to the relays are damaged.